The secret memoirs of Zhao Ziyang, the Communist Party leader ousted for opposing the military crackdown on student protesters in Tiananmen Square, exploded into the open yesterday, four years after his death.

Dictated during his years of house arrest and smuggled out on cassettes disguised as children's music or Peking opera, the book will be pored over for clues about the workings of the secretive group of men who make up the inner core of China's Communist Party. The decisions made in Beijing's Zhongnanhai compound have global impact as China is an emerging superpower, but little is known about how it functions. Prisoner of the State: The Secret Journal of Zhao Ziyang may change all that.

The publishers, Simon and Schuster, were so worried about news of the Zhao book leaking that they listed it as Untitled by Anonymous in their catalogue. It was not supposed to go on sale until next Tuesday but several stores in Hong Kong broke the embargo and put it on the shelves. And the clamour – just ahead of the 20th anniversary of the 4 June Tiananmen Square massacre, when tensions are high about political dissent in China – was intense.

Mr Zhao was a powerful figure within China's opaque apparatus of power, but his decision to back the student protesters in Tiananmen Square cost him his career, and earned him 16 years under arrest in his Beijing home. His last public appearance was on 19 May, 1989, when he visited the young demonstrators in front of the Forbidden City and urged them to leave Tiananmen Square, warning that police would use force if they did not. Standing beside him was his aide, Wen Jiabao, who escaped the taint of his allegiance to Mr Zhao to become the current premier.

As the tanks rolled into central Beijing on 3 June Mr Zhao writes: "While sitting in the courtyard with my family, I heard intense gunfire. A tragedy to shock the world had not been averted, and was happening after all."

The current Chinese leadership says the crackdown was a "disturbance" by "hooligans" and says crushing the revolt was essential to ensure a stable foundation for the country's economic growth. Mr Zhao takes the opposite view. "I had said at the time that most people were only asking us to correct our flaws, not attempting to overthrow our political system," he wrote.

There are lively examples of his rivalry with the veteran revolutionary and former supreme leader Deng Xiaoping. Mr Deng is hailed in China as the architect of the last 30 years of reform and economic liberalisation. However, Mr Zhao paints a very different picture, one of a double-crossing and cunning political leader at odds with the official hagiography.

Mr Zhao – who was the general secretary of the Communist Party from 1987 to 1989 – says by removing him from power, Mr Deng and others had simply ignored their own rules meant to prevent a return to the cult of personality that characterised the Mao years. The decision was made without a vote in the Politburo and the memoir describes in gripping detail how Mr Deng summoned the Standing Committee to his house to purge Mr Zhao.

"Reading Zhao's unadorned and unboastful account of his stewardship, it becomes apparent that it was he rather than Deng who was the actual architect of reform," writes Roderick MacFarquhar, professor of Chinese history at Harvard University.

Mr Zhao – the son of wealthy landowner who was purged as a capitalist sympathiser during the Cultural Revolution before being rehabilitated – believed the cure for China's problems lay in gradual but unceasing movement towards Western-style democracy, something the current leadership has ruled out.

The book is being published in Chinese by New Century Press, which is run by Bao Pu, the son of Mr Zhao's most trusted aide Bao Tong who remains under police surveillance. Bao Tong yesterday shrugged off suggestions that the leaked memoirs could split the Communist Party but said senior leaders were likely to read the book. "It will give them a lot to think about, and cause them to think about the Party's basic survival," he told Reuters.

In a review of the book, Perry Link, a leading commentator on Chinese politics, noted how incarceration had given Mr Zhao time to reflect on China's place in history. "At the end, we see Zhao arrive at positions more radical than any he had taken before – positions that the Chinese government had long been calling 'dissident'... The Communist Party will have to release its monopoly on power. Ultimately, China will need parliamentary democracy," Mr Link writes.

Mr Zhao died a lonely old man in his Beijing house in 2005. "The entrance to my home is a cold, desolate place." But his astonishing memoir may mean his life takes on a whole new significance.

China 20 years ago: Students vs the party

The 1989 Tiananmen Square massacre was described as "a tragedy to shock the world" by Zhao Ziyang, and a "disturbance" instigated by "hooligans" by the Chinese regime. The image of a lone man standing in front of a row of tanks remains an enduring image of the incident, which sprang from protests over the government's refusal to allow public mourning upon the death of pro-democracy official, Hu Yaobang.

The protest became a proxy for more generalised anger at the government's authoritarianism. Over seven weeks, the crowd swelled to over 100,000 students and workers. A hunger strike led authorities to take brutal steps to suppress the protests, and tanks drove the students from the square – killing 2 to 3,000 people in the process.

Prisoner of State: Extracts from the book

On the 17 May meeting "I walked out as soon as the meeting adjourned. At that moment, I was extremely upset. I told myself that no matter what, I refused to become the General Secretary who mobilised the military to crack down on students."

On the Tiananmen crackdown "On the night of June 3rd, while sitting in the courtyard with my family, I heard intense gunfire. A tragedy to shock the world had not been averted, and was happening after all... First, it was determined then that the student movement was a planned conspiracy of anti-Party, anti-socialist elements with leadership. So now we must ask, who were these leaders? What was the plan? What was the conspiracy? What evidence exists to support this? Second, it was said that this event was aimed at overthrowing the People's Republic and the Communist Party. Where is the evidence? I had said at the time that most people were only asking us to correct our flaws, not attempting to overthrow our political system."

On democracy "It would be wrong if our Party never makes the transition from a state that was suitable in a time of war to a state more suitable to a democracy society... The ruling Party must achieve two breakthroughs. One is to allow other political parties and a free press to exist. The second... is, the Party needs to adopt democratic procedures and use democratic means to reform itself... Different opinions must be allowed to exist, and different factions should be made legitimate."

The last word "Whether the Communist Party persists should be determined by the consequences of society's political openness and the competition between the Communist Party and other political powers (...) The trend is irrefutable, that the fittest will survive."